It is known that high-order mode filters can reduce higher modes to prevent optical elements having optical waveguides from being degraded in performance due to inclusion of higher modes and leaking modes (hereinafter, referred to as “higher modes”). Conventionally, it is known that taper-structured waveguides can remove higher modes by use of very thin single-mode waveguides. However, taper-structured waveguides cause higher modes due to tapered structures by themselves. It is difficult to produce taper-structured waveguides using extremely thin widths of waveguides because wherein they suffer from a drawback in that the lengths of elements are increased to be relatively longer because a basic-mode loss increases due to processing roughness on side faces of waveguides.
Patent Literature Document 1 discloses a waveguide-type high-order mode filter which can solve the problem of a taper-structured waveguide. Such a high-order mode filter applies diffraction grating on side walls so as to effectively scatter higher modes while achieving a small basic-mode loss. Patent Literature Document 1 discloses an example of a calculation regarding a high-order mode filter with a basic-mode loss of 1 dB and a high-order mode loss of 10 dB. The high-order mode filter is not applicable to high-accuracy optical communication technologies, which require small losses in optical waveguides.
To solve the above problem, it is possible to provide a high-order mode filter with any means that can efficiently and solely remove higher modes in laser elements. For example, Patent Literature Document 2 discloses a laser element in which a light-wave reducing loss element is disposed at a predetermined position in an optical waveguide. A light-wave reducing loss element is disposed at the position at which a higher mode is influenced under coherent superposition of a basic mode and a higher mode, thus efficiently removing a higher mode.
Patent Literature Document 3 discloses an optical filter which can remove a higher mode in an optical waveguide. The optical filter is a high-order mode filter with a small basic-mode loss, which will be described with reference to FIG. 12(a), (b). A high-order mode filter 100 is configured by alternately connecting a first waveguide 104 with a small terrace width and a second waveguide 105 with a large terrace width in a multi-mode rib waveguide in which a projection 101 is disposed on a terrace 102, thus efficiently removing a higher mode alone. Compared with a basic mode, a higher mode may cause a large expansion of an electric field over the terrace 102, thus causing a high loss at a connecting face 111 between the first waveguide 104 and the second waveguide 105. In contrast, a basic mode may cause a small expansion of an electric field over the terrace 102, and therefore it is possible to reduce any loss at the connecting face 111.